In flat-panel displays such as a liquid crystal display and the like, a protection panel for protecting a display panel, which contains a transparent plates such as an acrylic panel, a glass plate and the like, is conventionally set with a certain gap from the display panel, so that when some impact is inflicted thereon, the impact will not reach the display panel.
However, since the gap is generally an air layer, a light reflection loss is produced due to the difference in the refractive index between the above-mentioned display panel or the material constituting the protection panel, and the air layer, which problematically prevents good visibility.
Hence, for example, a technique for eliminating a clearance (air layer) between a liquid crystal panel and a protection panel in liquid crystal displays is known, which includes bonding a liquid crystal panel to a transparent protection plate via a transparent adhesive sheet made of an acrylic adhesive (e.g., acrylic ester copolymer crosslinked with an epoxy, isocyanate, melamine or metal compound crosslinking agent, a UV curable acrylic adhesive and the like, which have been processed into sheets) (JP-A-2002-348546 etc.). Moreover, for a similar purpose, use of an adhesive made of a polyorganosiloxane composition having a specific plasticity (silicone adhesive) and processed into a sheet has also been proposed (JP-A-2004-212521).
As mentioned above, however, when a liquid crystal panel is adhered to a transparent protection plate via a transparent adhesive sheet in liquid crystal displays, it is not easy to adhere the liquid crystal panel to the transparent protection plate without displacement, and further, without a void between the liquid crystal panel or transparent protection plate and a two-sided adhesive sheet, which void causing a light reflection loss that degrades display visibility, and such inconvenience cannot be completely eliminated. Moreover, even if air bubbles are not enclosed during adhering by visual observation, it sometimes occurs that minute void components originally enclosed in an adhesive sheet move to the interface between the adhesive sheet and a display panel or transparent protection plate over time and form a void causing a light reflection loss, whereby the display visibility is reduced. Since silicone and acrylic transparent adhesive sheets after adhesion to a transparent protection plate or a liquid crystal panel are difficult to detach, once the above-mentioned inconvenience occurs, the expensive display panel and transparent protection plate need to be discarded. In view of such problems, therefore, the Applicants of the present application proposed a polyoxyalkylene adhesive sheet as a transparent adhesive sheet superior in redetachability, which can be stuck again (JP-A-2008-266473). Meanwhile, portable instruments with display function such as portable telephone, Personal Digital Assistant (PDA), handheld game machine, car-navigation system and the like are progressingly designed thinner at a remarkable speed in recent years, which necessitates reduction of the thickness of flat-panel displays to be mounted thereon such as a liquid crystal display and the like. As a result, for example, a display panel of a flat-panel display to be incorporated into such instruments with portable display function is designed still thinner and when, for example, a transparent protection plate of a display panel is a glass plate, a plate with a thickness of less than 1.0 mm is considered for use. However, when such a thin display panel is bonded to a transparent protection plate via the aforementioned polyoxyalkylene adhesive sheet, since the display panel and the transparent protection plate are poor in flexibility, they cannot be detached from each other without causing a breakage or crack. In flat-panel displays to be mounted on instruments with portable display function, a touch panel may be attached to a display panel. A transparent protection plate may be bonded to a glass plate with a transparent electrode, which is an operation plate of the touch panel, via a transparent two-sided adhesive sheet. In this case, also, the glass plate with a transparent electrode and the transparent protection plate problematically cannot be detached from each other without causing a breakage or crack (cannot be detached reusably). Recently, moreover, it has been proposed to form a layer by filling a highly transparent curable resin such as acrylic, urethane acrylate series, silicone series and the like, which is cured by heat or UV irradiation, between a display panel and a transparent protection plate to integrally adhere them. However, since such acrylic, urethane acrylate series and silicone series curable resin layers, which are cured by heat or UV irradiation, also adhere to a display panel and a transparent protection plate with a comparatively high adhesive strength, the transparent protection plate cannot be easily detached without causing a breakage or crack when the display panel and the thin transparent protection plate need to be re-bonded (re-adhered).